1. Field of the Invention
The present invention relates to a method of storing molecules whose standard state under ambient conditions is as a gas. More specifically, the present invention relates to a method and system of storing hydrogen and its isotopes by absorption and/or mixture within a fluid medium, such as a fluid mixture for use as a fuel, energy storage, or chemical applications.
2. State of Technology
There is considerable interest in replacing fossil fuels with hydrogen because of hydrogen's high energy density per unit weight, its ready availability through the electrolysis of water, and the absence of polluting byproducts from its use. A number of technological components present challenges in making this transition to a hydrogen economy, and in the development of appropriate systems and infrastructure that can integrate into those that already exist.
In the hydrogen economy, hydrogen is targeted to be stored in different places, in different unit volumes, and in operationally varying configurations, as it moves down the supply chain from producers to consumers. Producers may need to store large inventory volumes. Hydrogen may be stored in transporting vessels as it travels from producers to distributors. Fuel distributors, including stations that deliver fuel for vehicles, other power-driven devices, and electronic devices, often may need large quantities on hand. Small point-of-use storage containers are destined to be required in power plants, vehicles, and personal electronics. All these hydrogen storage applications have in common the need to safely maximize the amount of hydrogen stored per unit of storage system volume, and differ fundamentally from the gasoline distribution system in which the fuel retains the same and incompressible form throughout the supply chain.
The standard methods of hydrogen storage are in the form of a gas compressed under high pressure or a liquid maintained at cryogenic temperatures. Safety, both real and perceived, is an often-raised criticism of high pressure hydrogen storage as a compressed gas, wherein such a method of hydrogen storage has historically been limited by the intrinsic compressibility of hydrogen gas and the strength of pressure vessel materials resulting in bulky, heavy and/or relatively costly hydrogen storage vessels.
Storage of hydrogen in the form of liquid hydrogen (LH2) has therefore been a favored method of bulk storage and transportation of hydrogen under low pressure in lightweight and compact containers. However, hydrogen has the second lowest boiling point of any substance (20 K), making hydrogen liquefaction exceptionally complicated and energy intensive, requiring electricity equivalent to 30–40% of the fuel energy value of hydrogen. This low boiling point has also made evaporation of H2 from small LH2 tanks a difficult problem. Even after 20 years of development, the best vacuum-insulated automotive LH2 tanks begin to vent hydrogen vapor after only a few days to relieve pressure buildup as heat flow into the tank from the environment warms the liquid hydrogen.
Storage of hydrogen in solid form by (reversible) chemical reaction with metals to create metallic hydrides has also been employed. Hydride materials typically have high theoretical hydrogen storage densities, but achieve only about 50% volumetric efficiency as hydride powders expand upon reaction with and absorption of hydrogen gas and can require heat exchange equipment. Hydrides also permit relatively low-pressure hydrogen storage, but rapid refueling requires increased pressures to overcome the heat resulting from the absorption of hydrogen gas and the exothermic reaction with the metal to form the metal hydride. Typical metallic hydrides are either relatively heavy (e.g. iron-titanium and lanthanum-nickel based hydrides) or have high decomposition energies requiring very high temperatures (e.g. magnesium hydride) to release hydrogen.
Hydrogen can also be stored by reversible chemical reaction with liquids. Aromatic molecules with carbon-carbon double bonds are the leading candidates for chemical storage of hydrogen in liquid form. A methylcyclohexane molecule (C7H14) for example, releases 3 H2 molecules and becomes toluene (C7H8) when heated to temperatures as high as 650 K. Such a reaction has a theoretical reversible hydrogen storage density about 50 kg H2/m3 and capacity of 6 wt % H2.
An alternative to chemical hydrogen storage (e.g., as solid metal hydrides or liquids) is adsorption of H2 molecules onto lightweight high surface area solid adsorbents, such as carbon. Initial, typically cryogenic, H2 adsorption research on high surface area carbons began in the 1960's and continued through the 1990's. The benefits of this approach decline with increasing pressure; however, as the volume of the adsorbent itself occupies volume available to hydrogen gas of ever higher densities. At pressures above about 200 atmospheres, removing the adsorbent usually increases hydrogen storage density at cryogenic temperatures. Finally, the physisorption of H2 molecules onto an adsorbent surface is typically exothermic, complicating rapid refueling, especially under cryogenic conditions.
With the discovery of C60 and related structures, carbon materials engineered on the atomic scale have been studied and proposed as H2 adsorbents. For example, graphite nanofibers are a class of engineered carbon materials that have received significant attention with experimental claims of extraordinary H2 storage densities. In addition, carbon nanotubes have indicated some potential to adsorb H2 near room temperature but the current understanding of H2 adsorption within such carbon nanotubes (or other engineered adsorbents) is still embryonic. The energetic and economic manufacturing costs of such advanced solid adsorbent materials are also currently unknown.
Accordingly, a need exists for a lightweight medium with reduced and/or eliminated evaporative hydrogen losses in a form that permits storage systems to operate at predetermined pressures lower than those presently adapted for high-pressure hydrogen gas storage and temperatures less extreme than those presently adapted for liquid hydrogen (LH2) storage. The present invention is directed to such a need.